The present invention relates to a signal combining system for diversity reception.
Signal combining systems for diversity reception are classified into three different types, i.e., the selection type, linear combining (or equal-gain combining) type and non-linear combining type. Each of these systems is further classified into the pre-detection type and the post-detection type. Among these systems, the non-linear combining system, also known as the ratio squarer combining or maximal (or optimum) ratio combining system, is the most efficient, and many circuit structures of this system have been proposed and made available for practical use.
The maximal ratio combining of pre-detection signals requires phase control to keep signals received on diversity channels in phase with each other and amplitude control to weight voltage ratios between the received signals. Techniques of the prior art can be roughly classified into the following two categories from the viewpoints of phase control and amplitude control:
(1) The phase control is achieved with a phase lock loop while amplitude control is achieved via an envelope detection circuit and a multiplier, wherein the amplitude controlled is proportional to the square root of the autocorrelation of the input signal;
(2) Both phase and amplitude control is achieved with a regenerative feedback loop, wherein the weights for the amplitude control are the correlations between the normalized combined output signal and the received signals on individual channels. For the details of the technique of item (2), reference is made to J. R. Sharman, "Pre-detection Combining" in Point-to-Point Communication, VOL. 17, No. 3, published by Marconi Communication Systems Limited, September 1973.
Both these techniques always require, for combining predetection signals, heterodyne frequency conversion which never fails to shift the center frequency of a pre-detection signal, such as from a radio frequency into an intermediate frequency or from a first intermediate frequency into a second. This necessitates the preparation of a device for each signal frequency band, resulting in an economic disadvantage, apart from which the two systems have the following shortcomings.
The system of item (1) usually employs, for phase control, a voltage-controlled crystal oscillator (VCXO) in its phase lock loop. This VCXO, because its center frequency varies due to aging or other causes, may give rise to the unlocking of the loop, therefore involving a stability problem. The phase lock loop has to repeat the locking operation every time each channel is restored after a fade-out. In both cases, the unlocking phenomenon adversely affects reliability. Moreover, this system requires a detection circuit, multiplier and the like for amplitude control, separately from the circuit for the phase control.
On the other hand, the system of item (2) requires a narrow band-pass filter in its regenerative feedback loop. This narrow band-pass filter has to be a high-precision device such as a crystal filter for a pre-detection signal. Also, since this system, as stated above, requires the heterodyne frequency conversion, the succeeding demodulator and other components have to be suitable for a second intermediate frequency, which is different from the intermediate frequency on the modulation side. Hence, when a modulation-demodulation loop back test within a station is performed, a particular device is required for the heterodyne frequency conversion, resulting in inconvenience.
An object of the present invention therefore is to provide a simpler and more practical pre-detection signal maximal ratio combining system for diversity reception requiring no heterodyne frequency conversion, unlike similar systems of the prior art.
In accordance with the invention, there is provided a system for pre-detection maximal ratio combining of a plurality of received diversity signals, comprising: a plurality of control means for controlling the phase and amplitude of each of a plurality of pre-detection signals of the received diversity signals by correlating the mutually orthogonal components of each of the pre-detection signals with a reference comparison signal; means for combining the outputs of the plurality of control means; and means for normalizing the amplitude of the combined signal and respectively supplying this normalized signal, as the reference comparison signal, to the plurality of control means.